1. Field of the Invention
This invention relates to an improvement in or relating to radial tires, and more particularly an improvement in a bead structure of a radial tire comprising polyester cord carcass plies one edge of which is folded back around a bead core, and a steel chafer of reinforcing metallic cords provided outside the folded edges of the carcass plies and further comprising a reinforcing layer of organic fiber outside the steel chafer.
2. Prior Art
In comparison with steel cords, polyester cords undergo less deterioration in strength and adhesion due to moisture and enjoy a higher degree of fatigue resistance. In addition, the polyester cords produce a smaller tire growth than nylon cords. Thus, polyester cords are most desirable as carcass cord material.
Several carcass ply structures are well known: (1) all carcass plies are folded from the inside of the bead core to the outside (FIG. 1A); (2) some carcass plies are terminated at the inside or lower end of the bead core and the other carcass plies are folded from the inside to the outside of the bead core (FIG. 1B); (3) some carcass plies are folded from the inside to the outside of the bead core and others from the outside to the inside (FIG. 2); and so forth. The former two structures (FIGS. 1A and 1B) are easier to manufacture and more popular than the latter structure (FIG. 2), whereas the former structures are inferior to the latter in rigidity at the head region (FIG. 2) and even to the same structure using steel cords in the same way. Since those tires are subject to greater deformation in use, stress concentration occurs at the folded edges of the carcass plies and at the upper end of the steel chafer.
Elongation of the cords within the carcass plies due to tension exerted thereupon during use of the tires is approximately 0.5% when steel cords are used in the carcass plies, and approximately 5% when polyester cords are used. Therefore, as denoted by the arrow in FIG. 3, the strain occurring between the folded edges of the carcass plies or the upper end of the steel chafer and the peripheral rubber is increased. The stress concentration and strain tend to cause separation at the folded edges of the respective carcass plies and the upper end of the steel chafer.
As measures to avoid those problems, the bead region can be made of hard rubber or the folded edges of the carcass plies can be terminated at an elevated level above the upper end of the steel chafer. However, these measures fail to provide the required durability. Because, the respective carcass plies contain a substantial number of cords to maintain toughness of the carcass plies, a large number of cord ends are present at the cross section of the folded edges of the carcass plies. The cross section of the folded edges are not subjected to chemical treatment, and this is one of the reasons causing separation. Concerning the structure where the folded edges of the carcass plies are terminated at an elevated level above the upper end of the steel chafer, the folded edges of the carcass plies are located in a region subjected to great load deformation during use of the tire, and this is another cause of separation. Moreover, since the folded edges of the carcass plies are located at an angle of approximately 90.degree. relative to the circumferential direction of the tires, it is impossible to absorb deformation while a load is on the tires and the folded edges act to peck the peripheral rubber material, thereby causing and promoting separation.
It is, therefore, desirable to terminate the folded edges of the carcass plies at a region below the upper end of the steel chafer where strain is comparatively small. The upper end of the steel chafer is a free end and has less rigidity and particularly bending rigidity than the central portion thereof, the rigidity being increased gradually from the top to the center. To use successfully such properties of the steel chafer, it is necessary to determine the height of the folded edges of the carcass plies and the height of the steel chafer, taking into account the relative position between the folded edges of the carcass plies and the steel chafer, and the separation resistant properties of the steel chafer at its upper end.
In addition, separation will occur at the upper end of the steel chafer in used radial tires which are used subjected to heavy duty use for a long period of time, for example, radial tires for truck or bus use. In other words, a rigidity differential is necessarily developed about the upper end of the steel chafer and causes separation easily. Moreover, the upper end of the steel chafer itself is free and easily movable due to load deformation during tire use, thereby further enhancing the possibility of causing and promoting separation.